(1) Field of the Invention
The present invention relates to a method and device for controlling bias of optical modulator, and more particularly to a method and device for controlling bias of optical modulator for adequately controlling the DC bias of each of a plurality of optical modulating sections of an optical modulator comprising the plurality of optical modulating sections therein.
(2) Related Art Statement
Optical modulator is frequently used as an electrical-optical conversion device in the field of optical communication or optical measurement.
An optical intensity modulator comprising a substrate with an electro-optic effect such as LiNbO3 is cited as an example of the optical modulator. However, such optical modulator is known for causing a so-called drift phenomenon that the output characteristics of light change with time due to the amount of the applied DC bias for drive control or due to the temperature change of its operating environment.
As the method for controlling such drift phenomenon, the following patent documents 1 or 2 discloses a method for superposing a low frequency electrical signal on the driving signal of an optical modulator, monitoring the change of light intensity related to said low frequency electrical signal included in the output light from said optical modulator, and detecting a bias point against the actual applied voltage, and further combining the method with a bias compensating circuit for controlling a DC bias to be applied into the optical modulator to enable automatic correction of optical response characteristics so as to make the most appropriate bias point.
[Patent Document 1] Japanese Patent Laid-open No. S49-42365
[Patent Document 2] Japanese Patent Laid-open No. H03-251815
On the other hand, an optical communications system capable of high density, high speed and long distance transmission is required corresponding to the increasing demand for telecommunications. Especially, it is necessary to develop a DWDM communications system. For this DWDM communications system, it is necessary to solve objects of increasing usability of frequency, increasing of nonlinear effect resistance (long distance telecommunications), or the like. The present applicant has proposed Single Side-Band (SSB) modulator as the modulator that excels in those characteristics.
One example of the SSB modulator is disclosed in the following non patent document 1.
[Non Patent Document] “X cut LiNbO3 Optical SSB-SC modulator” (HIGUMA, Kaoru et al., p 17 to 21, “Technical Report 2002(Sumitomo Osaka Cement Co., Ltd.)”, issued by Sumitomo Osaka Cement Co., Ltd, New Technology Research Center on Dec. 8, 2001)
The operation principle of the SSB modulator is explained.
FIG. 1 is a diagram showing a frame format of the optical waveguide of the SSB modulator, especially a Single Side-band with Suppressed Carrier (SSB-SC) modulator.
The optical waveguide as FIG. 1 is formed by diffusing Ti on a substrate with an electro-optic effect such as LiNbO3. Said optical waveguide comprises nesting MZ structure which has two sub MZ (Mach-Zehnder) waveguides MZA and MZB placed in parallel in each arm of main MZ waveguide MZC.
RFA and RFB show simplified traveling-wave-type coplanar electrodes for applying the modulating signal of a microwave into sub MZ waveguides MZA and MZB. In addition, DCA and DCB show simplified phase adjustment electrodes for applying direct voltages into sub MZ waveguides MZA and MZB, and DCC shows a simplified phase adjustment electrode for applying a direct voltage into main MZ waveguide MZC in order to provide them with a predetermined phase difference.
The principle of an SSB modulator without suppressed carrier is explained before explaining the operation in FIG. 1. SSB technology has been used in the area of wireless communication. It is known that SSB modulating signal is obtained by summing original signals and signals which are converted by using Hilbert conversion.
SSB modulation without suppressed carrier is obtained by using the dual driven single MZ modulator as shown in FIG. 2 (the example using Z cut substrate is shown in the figure).
Incident light as exp(jωt), single frequency RF signal, φ cos Ωt, and a signal, converted by Hilbert conversion, H[φ cos Ωt]=φ sin Ωt are simultaneously input from RFA port, and RFB port, respectively.
Because sin Ωt=cos(Ωt−π/2), both signals can be input simultaneously by using a phase converter for microwave. However, φ, ω, and Ω mean modulation, optical wave, and frequency of microwave (RF) signal, respectively.
Moreover, phase difference π/2 is given to optical waves which pass through both arms of the MZ waveguide adding appropriate bias from DCA port.
Consequently, the formula focusing on the phase term of optical wave at multiplexed point becomes as the following formula (1).exp(jωt)*{exp(jφ cos Ωt)+exp(jφ sin Ωt)*exp(jπ/2)}=2*exp(jωt)*{J0(φ)+j*J1(φ)exp(jΩt)}  (1)
Here, J0 and J1 are 0 and primary Bessel function and components after secondary are ignored.
As in the formula (1), 0 and primary spectrum remains, however, components at −1(J−1) have been lost (When this is typically shown, the optical waves with spectrum distribution on the right side of the MZ waveguide in FIG. 2 exit from the MZ waveguide).
In addition, a bias adding a phase difference of −π/2 is applied into DCA port to retain components at −1(J−1) and delete primary components (J1).
Subsequently, the Single Side—Band with Suppressed Carrier (SSB-SC) modulator has sub MZ interference systems added to both arms of single MZ interference system as shown in FIG. 1.
The signals as shown in FIG. 3 are applied into these sub MZ waveguides. This can be considered to be the same as normal intensity modulation performed by bottom drive.
Here, the following formula (2) is the phase term of outgoing light.exp(jωt)*{exp(jφ sin Ωt)+exp(−jφ sin Ωt)*exp(jπ)}=2*exp(jωt)*{J−1(φ)exp(−jΩt)=J1(φ)exp(jΩt)}  (2)
The above formula explains that even number spectrum components including carrier components are cancelled (When this is typically shown, the optical waves with spectrum distribution on the right side of the MZ waveguide in FIG. 3 exit from the MZ waveguide).
Then, by combining the above mentioned SSB modulation (formula (1), the modulation method shown in FIG. 2) and the carrier suppression method at sub MZ (formula (2), the modulation method shown in FIG. 3), it becomes possible to selectively generate either primary spectrum (J1 term) or −1 spectrum (J−1 term).
The frequency of primary spectrum light indicated by J1 is ω+Ω, and the frequency of −1 spectrum light indicated by J−1 is ω−Ω. This means that the light (frequency ω)) entering the SSB modulator is wavelength-shifted only for the frequency of the microwave applied into the SSB modulator, and is emitted as an outgoing light (frequency ω±Ω).
As stated above, the SSB modulator can be used as a wavelength converter. In particular, the SSB-SC modulator can prevent the generation of 0 spectrum and generate primary or −1 spectrum effectively.
The optical modulator with three combined MZ waveguides as shown in FIG. 1 is called nesting type optical intensity modulator (OSSBM, Optical Single Side-Band Modulator) in particular.
Although various types of multifunction and high-performance optical modulators, each of which comprises a plurality of optical modulating sections therein, as stated above have been proposed, it is necessary to perform DC bias correction related to driving of the optical modulating sections to maintain an appropriate drive bias point because the drift phenomenon could occur at any time in the optical modulator having a substrate with an electro-optic effect as described above.
If the method for controlling drift phenomenon concerning the optical modulator as stated above is used, it becomes necessary to control three DC biases DCA, DCB and DCC in the case of the nesting type optical intensity modulator in FIG. 1, for example. Further, in order to correct and control DCA and DCB, it is necessary to separately provide a detecting means for detecting optical waves passing through sub MZA and MZB. Even if this is not provided, it is necessary, when DCA is controlled for example, to make the other MZ sections (MZB, MZC) not operate, or the like.
As stated above, configuration related to control of DC bias of optical modulator gets complicated in accordance with the increasing optical modulating sections. Further, there occurs the problem that correction is not possible in stationary optical communication or optical measurement because, when the input/output characteristics of a specific optical modulating section are measured, the other optical modulating sections are made not to operate.
The present invention intends to solve the above mentioned objects and to provide a method and device for controlling bias of optical modulator capable of adequately correcting the direct current bias of each of a plurality of optical modulating sections of an optical modulator comprising the plurality of optical modulating sections even while the optical modulator is operating in normal mode and even with a simple structure.